Method and apparatus for weeding the flame plasma of a rocket exhaust



Deco 1966 w. w. BALWANZ 3,289,410

' METHOD AND APPARATUS FOR WEEDING THE FLAME PLASMA OF A ROCKET EXHAUSTFiled June 17, 1964 22 2E r M .1 1

Qo E j 99 LLLL 3% m4 mm 22 2E N 33 (5 JJ I :9 E; 33 LLLL 1 Eu. L I 1,57381930 NI 8801 \VNENS INVENTOR WM. 4'. M? M W BALWA IVVZ' c g X-WJSZA/ BY r. AGENT ATTORNEY United States Patent 3,289,410 METHOD ANDAPPARATUS FOR WEEDING THE FLAME PLASMA OF A ROCKET EXHAUST William W.Balwanz, Alexandria, Va., 'assignor to the United States of America asrepresented by the Secretary of the Navy Filed June 17, 1964, Ser. No.375,989 Claims. (Cl. 60205) The invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

The present invention relates to a method and apparatus for weeding orreducing the ionizations in hot exhaust gases. More particularly, theinvention relates to a plasma weeding method which is capable ofminimizing exhaust interference with communication links.

Rocket aircraft in flight relay information by radio to the ground. Inthis case and also when radar methods are used to track the vehicle, orwhen it is controlled by radio signals from a ground-based guidancesystem, there may be considerable interference with the signal due tothe electromagnetic properties of hot exhaust gases. The combustionprocess in rocket motors evolves free electrons, free radicals and othertransient species that form flaming trails of highly ionized media.Electromagnetic radiation passes through an un-ionized gas, such as air,without interference. When the gas is partly ionized, howeverabsorption, reflection and refraction of the transmitted radiationresults in a considerable loss of signal and therefore decreasedreliability in the guidance and tracking systems.

Absorption and phase shift experienced by the electromagnetic radiationdepend on the concentration of free electrons formed by thermalionization of exhaust gases, the electron concentration increasingrapidly with temperature. A typical concentration would be aboutelectrons per cubic centimeter. Electrons are also emitted from hotsolid particles of soot or metal in the exhaust, just as they areemitted from the heated cathode of a vacuum tube. If the propellant isfuel rich, afterburning will take place when the exhaust mixes with thesurrounding atmosphere, and this too causes further increase inionization.

It has previously been proposed to include certain materials in thefuel-oxidant mixtures to improve the combustion process and modify thecomposition of the exhaust gases. Extensive experimentation has provedthat it is very difficult to alter temperature or composition of thepropellant gases without incurring a substantial reduction inperformance. Other methods have been advanced in the past for removingelectrical charges from hot exhaust gases by applying electricalcollection or deflection probes, but these devices were whollyineffective for weeding high electron densities from a flame plasma.

The term flame plasma refers to the limited degree of ionization whichexists in hot exhaust gases in which the gaseous state containsapproximately equal densities of charged carriers of oppositepolarities.

The present invention concerns an effective weeding method for reducingthe ionization of a rocket exhaust to a level where it can not interferewith radar and radio signals. More specifically, the present methodreduces the electron density in the plasma by means of a weedersubstance which is introduced directly in-to the exhaust during theflight of the rocket craft. The weeder method described herein differsfrom previous methods dealing with electrical charge which were directedprimarily to the propellant properties during combustion. The weeder inaccordance with the invention is utilized in such a manner "ice that itdoes not interfere with the propulsion process, but is directed entirelyon the products of combustion as they escape from the rocket throat.

An object of the present invention is to provide a method which can beused on rocket-propelled craft or jet engines to suppress the exhaustflame.

Another object is to provide fluid injection means on a rocket for rapidmingling of a weeder substance with the combustion products from theengine.

A further object is to provide a method which minimizes exhaustinterference with radio and with radar probes.

A still further object is to provide a fluid dis charge element forintroducing a weeder substance near the exhaust nozzle of a rocket, saidsubstance being effective in reducing free electron charge near saidnozzle.

Other objects and advantages will be readily apparent from considerationof the following detailed description taken in connection with theaccompanying drawing in which:

FIG. 1 illustrates schematically a portion of a rocket engine whichincludes a plasma-weeding injector in accordance with the presentinvention, and

FIGS. 2 and 3 graphically illustrate the improvement in signaltransmittance by introducing a weeder substance into the exhaust flame.

In accordance with the invention, electron reduction is achieved in arocket exhaust by injecting one or more substances into the exhaustflame at a suitable flow rate. Substances which may be utilized for thispurpose are the Freon gases, water, water vapor and potassium sulfate.

Any additive which prevents the addition of energy to the plasma tendsto reduce the electron concentration by forming a radiative anddispersive medium capable of dissipating the energy. However, for thepurposes of the present invention, the compounds disclosed are capableof forming ionizable components with high bonding strength for freeelectrons. Compounds with a high electron attachment coeflicient canhelp reduce the electron density by producing negative ions whichinterfere negligibly with electromagnetic signals because of theirincreased mass. Weeder components are at a much lower temperature thanthe flaming gases thus tending to reduce the temperature of the exhaustby dilution. When the weeder is added as a liquid, the cooling effect ofvaporization also combines with the cooling effect by dilution toprovide a further reduction in temperature. Thus, any weeder compound ifinjected in sufficient quantity may reduce the exhaust ionization.However, the basis on which plasma weeders have been selected depends ontheir ability to inhibit and retard chemical action in the flame and ontheir electron affinity for removing free electrons. These mechanismsprovide substances of practical utility at nominal dispensing rates forairflight applications. Other substances containing elements of highattachment coeflicients, for example the halogens, are not necessarilyeffective weeders for the most part in high temperature exhausts becausethey may induce chemical reaction and add heat to the system causinghigher thermal ionizations.

The weeder substances disclosed herein are preferably used at flow ratesnot exceeding about 20% the flow rates of the propellant mixtures. Asubstantial decrease in radiation losses is noted when the presentweeder compounds are used in amounts of about 4-20% based on thepropellant rate of consumption. Radiation losses by absorption,reflection, etc. which do not exceed about 2% of the incident beam areconsidered negligible insofar as interference with radar or radioreceivers is concerned. Plasma weeders add to the weight problem of arocket and therefore should be utilized preferably at flow rates ofabout 5% based on the propellant rate.

The Freon gases are particularly effective weeders, except in highhydrogen-content exhausts with temperatures above 2500 K.; the gaseousrelease of these compounds cause a rapid reduction in plasmatemperature. In addition, Freon compounds are capable of inhibiting theflame by reducing the flame combustion rate. Compounds which have beenfound to be particularly useful as weeders in accordance with thepresent invention are the dichlorodifluoromethane, CCl F thedibromodifluoromethane, CBr F and the bromotrifluoromethane, CBrF ormixtures of these compounds.

Water is a most effective weeder and may be used as a liquid ordischarged as steam vapor. The water molecule has a high cooling actiondue to its high specific heat; its high electron aflinity results in H Oand ion OH*+l-l formation. The reactions of the water molecule with freeelectrons may be indicated as follows:

Having a high ionization potential, the water molecule does not form ioncomponents, nor is it chemically reactive with most exhaust products.

The effectiveness of potassium sulfate as a weeder component may beexplained in terms of its elements which have a relatively low heat ofreaction and tend to take heat away from the plasma. A potassium sulfatesolution, may combine the mechanisms involved in water and in thesulfate effectively to suppress free electron formation at even lowerrates of flow. Small quantities of the solid material may be included inthe rocket engine and dispensed by melting or ablating during exposureto the exhaust jet.

Referring now to FIG. 1, a schematic diagram of a rocket engine 11 ofconventional configuration shows a portion of a combustion chamber 12,the aft portion thereof converging to a narrow throat 13. The nozzle 14,through which the internally developed gases can escape, is shown as astraight expansion cone. A supply tank 15 is located in the fore regionof the engine away from the heat of the combustion chamber. The tankwhich stores the weeder substance is connected by a pipeline 16 to aninjection manifold ring 17 located at or near the throat region. Themanifold ring surrounds the engine externally to prevent interferencewith the cross section at the throat; said ring conveys the weeder fluidto various points in the throat or nozzle regions through a plurality ofoutlets or ducts 18 that pass through the engine wall. Force-operatedequipment such as a variable pump 22 that pumps the fluid through thepipeline in which the solution is controlled by a suitable pressureactivated control valve 23 through which the pump forces the fluid intothe nozzle region as a fine spray; the opening in said outlets controlthe amount of fluid injected into the nozzle region. The rocket exhaustflame 19 sweeps the fluid into the body of the flame. In view of theforce of the exhaust, the fluid should be introduced into the body ofthe flame to prevent surface sweep. Alternately, the weeder may beintroduced in the expansion cone, for example, from an injector ringsurrounding the cone region at point 21, in a further effort to improvethe penetration of the weeder into the flame region.

In the operation of a rocket-powered craft, the propellant consisting afuel and a fuel oxidizer when brought together in the combustion chambercombine exothermally in a temperature range from 2000 to 4000 K. and atchamber pressures of about 300 pounds per square inch to providepropulsion of the craft. After suflicient thrust has been created topush the craft off the ground, it rises above the earth and itspropellant burns and produces flaming exhaust gases. Signal loss due tothe flame is lessened appreciably by injecting a weeder in accordancewith the present invention. The weeder for example may be liquiddibromodifluoromethane or water sprayed into the exhaust stream. Theweeder is pumped at a constant flow rate and injected as a fine sprayfrom equally spaced outlets. The force of spray may induce shock waves,as demonstrated with water vapor in the example below. A dense liquidstream under force may induce shocks of such proportion that moreelectrons will be created than the weeder is able to remove.

Tests conducted with electromagnetic probes provide a measure of theelectron reduction which may be achieved by means of weeder action.FIGS. 2 and 3 demonstrate graphically the loss of electromagnetic energyincurred from ionizations in the exhaust flame of a 1500-pound thrustrocket motor, and the improvement that can be obtained by means ofweeder injections. The graphs also indicate the dependence of electronreductions on the concentration of the weeder substance. Electromagneticenergy was provided by square wave modulated klystrons to transmittingantennas. The energy received was rectilied and the video outputamplified and displayed on a cathode ray tube for photographicrecordings. Changes in the amplitude of this recorded pulse gave ameasure of the loss of energy due to the conditions of the exhaustflame. FIG. 2 demonstrates the energy loss in decibles before and duringthe injection of water to the exhaust flame. Prior to the weederinjection, the energy loss was about 37% of the incident wave beam. Whenthe water was turned ON at a rate of 1.7 pounds per minute which wasapproximately 17% of the propellant flow rate decreased the energy lossto less than 0.2%. When the water spray was turned OFF, the energy lossimmediately increased to the previous level. The horizontal axis on thegraph is the time parameter. FIG. 3 similarly demonstrates the decreasein energy loss resulting from the injection of steam at a lower rate offlow. The steam was injected at about 0.5 pound per minute orapproximately 5% of the propellant rate of flow. The improvement withsteam injection was less than in the previous test due to the lower flowrate and to shock ionization which created a stepwise increase inabsorption loss.

It will be apparent that the selection of weeding material and the rateat which it is introduced to an exhaust stream to obtain optimumtransmittance of electromagnetic signal will vary with rocket design andwith propellant composition. In practise, therefore, the weeder selectedand the amount injected into a flame must be based. on the flame plasmaexisting in a particular rocket exhaust.

Various modifications are contemplated and may obviously be resorted toby those skilled in the art without departing from the spirit and scopeof the invention. It is therefore to be understood that within the scopeof the appended claims the invention may be practised otherwise than asspecifically described.

What is claimed is:

1. A method for weeding the flame plasma of a rocket exhaust resultingfrom the combination of propellants in a rocket engine which comprises:

injecting a substance into the exhaust flame of a rocket engine withinthe exhaust nozzle thereof,

said substance being selected from the group of substances consisting ofCBr F CBrF and K 50 and being injected into the flame at a flow rate inthe range of from about 4 to about 20 percent based on the flow rate ofthe propellant.

2. A method for weeding the flame plasma of a rocket exhaust resultingfrom the combination of propellants in a rocket engine which comprises:

injecting a substance into the exhaust flame of a rocket engine within-the exhaust nozzle thereof,

said substance consisting essentially of CBr F and being injected intothe flame at a flow rate in the range of from about 4 to about 20percent based on the flow rate of the propellant.

3. A method for weeding the flame plasma of a rocket 5 exhaust resultingfrom the combination of propellants in a rocket engine which comprises:

injecting a substance into the exhaust flame of a rocket engine withinthe exhaust nozzle thereof,

said substance consisting essentially of CBrF and being injected intothe flame at a flow rate in the range of from about 4 to about 20percent based on the flow rate of the propellant.

4. A method for weeding the flame plasma of a rocket exhaust resultingfrom the combination of propellants in a rocket engine which comprises:

injecting a substance into the exhaust flame of a rocket engine withinthe exhaust nozzle thereof,

said substance consisting essentially of K 80 and being injected intothe flame at a flow rate in the range of from about 4 to 20 percentbased on the flow rate 'of the propellant.

References Cited by the Examiner UNITED STATES PATENTS 2,544,422 3/1951Goddard 6O35.6 2,916,873 12/1959 Walker 6035.54 3,188,801 6/1965 Prosser60-356 3,221,498 12/1965 Bankston 6035.54

MARK NEWMAN, Primary Examiner.

CARLTON R. CROYLE, Examiner.

D. HART, Assistant Examiner.

1. A METHOD FOR WEEDING THE FLAME PLASMA OF A ROCKET EXHAUST RESULTINGFROM THE COMBINATION OF PROPELLANTS IN A ROCKET ENGINE WHICH COMPRISES:INJECTING A SUBSTANCE INTO THE EXHAUST FLAME OF A ROCKET ENGINE WITHINTHE EXHAUST NOZZLE THEREOF, SAID SUBSTANCE BEING SELECTED FROM THE GROUPOF SUBSTANCES CONSISTING OF CBR2F2, CBRF3, AND K2SO4 AND BEING INJECTEDINTO THE FLAME AT A FLOW RATE IN THE RANGE OF FROM ABOUT 4 TO ABOUT 20PERCENT BASED ON THE FLOW RATE OF THE PROPELLANT.